1. Field of the Invention
Recently, mobile telephones for cellular systems and simple mobile phone systems (PHS) have become widely popular, and these mobile telephones are provided with antennas for the transmission and reception of call traffic and data traffic. Usually, for these antennas, a whip antenna is used, which is made of a retractable whip element that, for the sake of convenience when carrying the mobile telephone and anticipating a call, can be stowed away in a casing of the mobile telephone.
2. Description of the Related Art
However, when the whip element has been stowed away in the casing, transmission and reception with the mobile telephone are almost impossible, so that a small antenna element has to be provided outside the casing when the whip element is stowed away in the casing. Thus, this small antenna element can be used for transmission and reception when the whip element is stowed away in the casing.
A characteristic feature of helical antennas is that the physical length can be made shorter than the effective antenna length, so that helical antennas are used for the afore-mentioned small antenna elements. Conventionally known are configurations, in which the helical antenna is provided at the casing of the mobile telephone, or at the tip of a retractable whip element.
The configuration of such a conventional helical antenna is shown in FIGS. 10a and 10b. However, the helical antenna 100 shown in these drawings is of the type that is attached to the wireless device casing of a mobile telephone or the like, and the cover covering its outer surface is not shown.
FIG. 10a is a plan view of the helical antenna 100, and FIG. 10b is a cross-sectional view thereof. As shown in these drawings, a through hole 101a is formed in the insulating bobbin 101, which is made by resin casting, passing from the top to the bottom along the axis of the bobbin 101. The through hole 101a is provided for slidably inserting the whip antenna into it.
A single thread of helical protrusion portions 105 is formed on the peripheral outer surface of the bobbin 101. The helical protrusion portions 105 form a single thread, but for illustrative reasons, the numerals 105a to 105g are associated with the helical protrusion portions in FIGS. 10a and 10b. The helical protrusion portion 105g is taken as the leading end of the helical protrusion portions 105. The helical protrusion portions 105a to 105g form helical groove portions between them, and a helical element 122 is arranged in these helical groove portions. The helical element 122 is made of a wire of, for example, phosphor bronze, which is formed into helical shape. The helical element 122 is screwed from the upper end of the bobbin 101 into the helical groove, which leads to the situation shown in FIGS. 10a and 10b. 
The lower end of the bobbin 101 is provided with a threaded portion, and this threaded portion of the bobbin 101 is screwed into a threaded portion formed on an inner peripheral surface at the upper end of an attachment fitting 115. This fixes the attachment fitting 115 to the lower end of the bobbin 101. The lower portion of the helical element 122 that has been screwed onto the bobbin 101 is also wound around an upper cylindrical portion 115d formed in the upper portion of the attachment fitting 115, and the helical element 122 is in electrical contact with this upper cylindrical portion 115d. Furthermore, a through hole 115a connected to the through hole 101a is formed also in the attachment fitting 115. A collar portion 115b is formed in an intermediate portion of the peripheral side surface of the attachment fitting 115, and a threaded portion 115c is formed at a lower portion of the peripheral side surface. The threaded portion 115c is screwed to the wireless device casing until the lower surface of the collar portion 115b abuts against the wireless device casing, whereby the helical antenna 100 is attached to the wireless casing device. Thus, the attachment portion 115 connects the transmission/reception portion provided inside the wireless device casing with the helical element 122.
The helical antenna 100 with the conventional configuration shown in FIGS. 10a and 10b has a small outer diameter of several mm, so that the helical element 122 is provided with a wire diameter of about 0.6 mm. When this helically shaped helical element 122 is screwed from the upper end of the bobbin 101, it will be moved beyond the helical protrusion portion 105g, or the leading end and wound around the upper cylindrical portion 115d. Thus, the inner peripheral surface of the helical element 122 will be brought into contact with the upper cylindrical portion 115d. However, since the wire diameter of the helical element 122 is small, as mentioned above, and the screwing is performed manually, already small differences in the screwing force can lead to a vertical displacement of the position of the helical element 122 contacting the upper cylindrical portion 115d. Thus, when the position of the helical element 122 contacting the upper cylindrical portion 115d is shifted vertically, there are the problems that the effective number of windings of the helical element 122 changes, and the resonance frequency of the helical antenna 100 shifts.
This is explained with reference to FIGS. 11a, 11b, and 11c. As shown in FIG. 11a, the helical element 122g is in contact with the upper cylindrical portion 115d. The helical element 122g and the upper cylindrical portion 115d contact each other at the contact point 123 located on the right side in FIG. 11a. When the position of the helical element 122g is shifted slightly upwards as shown in FIG. 11b, then the helical element 122g and the upper cylindrical portion 115d contact each other at the contact point 123 located approximately at the center as shown in FIG. 11b. Furthermore, when the position of the helical element 122g is shifted even more upwards as shown in FIG. 11c, then the helical element 122g and the upper cylindrical portion 115d contact each other at the contact point 123 located on the left side in FIG. 11c. Thus, there is the problem that when the helical antenna 100 is used, for example, for the 900 MHz band, then this shifting of the contact point 123 causes a resonance shift of about 20 MHz for the shift shown in FIG. 11b, and a resonance shift of about 40 MHz for the shift shown in FIG. 11c. 
It is therefore an object of the present invention to provide a helical antenna, in which the helical element and the attachment fitting contact one another at a stabilized location.
In a helical antenna in accordance with the present invention, a leading end of the helical protrusion portions formed at the lower end of the bobbin has substantially the same diameter as the second collar portion, which is connected to this leading end, and the depth of the helical groove portions becomes gradually shallower toward this leading end, so that the helical element can be easily moved beyond the leading end and positioned on the side surface of the second collar portion. Since the outer diameter of the second collar portion and the outer diameter of the leading end are substantially the same and there is almost no difference in level between them, the helical element is better prevented from shifting vertically, and the helical element contacts the side surface of the second collar portion of the attachment fitting at a stabilized location. Consequently, deviation of the resonance point can be prevented.
When the helical protrusion portions including the leading end, which form the helical groove portion whose depth becomes gradually shallower, are formed higher, then this helical groove portion becomes deeper, and the position of the helical element within this groove portion is stabilized, so that the position of the helical element at the border between the bobbin and the attachment fitting is stabilized, and the position where the helical element contacts the attachment fitting is stabilized even better.
Furthermore, when the helical groove portion in an upper portion of the bobbin breaks off, and the helical element is screwed onto the bobbin from below until the upper end surface of the helical element abuts against the abutting portion where the helical groove portion breaks off, the bobbin can be screwed into the attachment fitting. Then, when the bobbin is being screwed into the attachment fitting, the upper end surface of the helical element is fixed by abutting against the abutting portion, so that the position where the helical element contacts the attachment fitting is stabilized even better, and deviation of the resonance point can be prevented even better.